1. Field of the Invention
The present invention concerns a rolled copper foil and, particularly, it relates to a rolled copper foil which has an excellent flexible fatigue property suitable for flexible wiring materials such as flexible printed circuits.
2. Description of Related Art
A flexible printed circuit (hereinafter simply referred to as FPC) has high freedom in a mounting form to electronic equipments due to their attractive features of small thickness and excellent flexibility. Accordingly, FPCs have been used generally, e.g., as wirings for bending portions of foldable (clamshell type) cellular phones, movable portions of digital cameras, printer heads, etc., and movable portions of equipment relevant to disks such as HDDs (hard disk drives), DVDs (digital versatile disks) and CDs (compact disks).
As conductors for FPC, pure copper or copper alloy foils (hereinafter simply referred to as “copper foils”) applied with various surface treatments have been generally used. The copper foils are classified into electrodeposited copper foils and rolled copper foils according to the manufacturing methods thereof. Since FPCs are used as wiring materials for repetitive movable portions as described above, excellent flexible fatigue properties (e.g., flexible fatigue property of 1,000,000 cycles or more) have been required, and rolled copper foils are often selected as the copper foils.
Generally, the rolled copper foils are manufactured by applying a hot rolling step to a cast ingot made of a tough pitch copper (JIS H3100 C1100) or an oxygen-free copper (JIS H3100 C1020) as a raw material, and then by repeating a cold rolling step and a process annealing step until a predetermined thickness. The thickness of rolled copper foils required for using in FPCs is usually 50 μm or less and it has tended to be decreased further as 10 and several μm or less in recent years.
The FPC manufacturing step generally includes “a step of bonding a copper foil and a base film (base material) comprising a resin such as a polyimide to form a CCL (copper claded laminate (CCL step))”, “a step of forming a printed circuit by a method such as etching for CCL”, “a step of applying a surface treatment on the circuit for protection of wirings”, etc. The CCL step includes two kinds of methods, i.e., a method of laminating a copper foil and a base material with an adhesive and then curing and adhering the adhesive by a heat treatment (3-layered CCL), and a method of directly bonding a copper foil applied with a surface treatment to a base material without an adhesive and then integrating them by heating and pressing (2-layered CCL).
In the FPC manufacturing step, copper foils as cold rolled (hard state which is work hardened) has been often used from a viewpoint of easy handling. In a case where the copper foil is in an annealed (softened) state, the copper foil is easy to deform (e.g., elongation, creasing, flexing, etc.) upon cutting of the copper foil or lamination with the base material, resulting in product failure.
On the other hand, the flexible fatigue property of the copper foil is improved remarkably by applying a recrystallization annealing than that of the copper foil in the as-cold rolled state. Then, a manufacturing method has been generally selected in which the heat treatment for adhering the base material and the copper foil in the CCL step is also served for the recrystallization annealing for the copper foil. The heat treatment condition in this case is usually at a temperature of 180 to 300° C. for 1 to 60 min (e.g., at 200° C. for 30 min) and the copper foil is in a state refined into a recrystallization texture.
For improving the flexible fatigue property of FPCs, it is effective to improve the flexible fatigue property of the rolled copper foil as the material thereof. Further, it has been known that the flexible fatigue property of the copper foil after recrystallization annealing is improved more as a cubic texture is developed. “Development of the cubic texture” referred to generally only means that the occupation ratio of a {200}Cu plane is high at the rolled surface (e.g., 85% or more).
Heretofore, for rolled copper foils with excellent flexible fatigue property and manufacturing methods thereof, there have been reported as follows. They are: e.g., a method of developing the cubic texture by increasing a final rolling working ratio (e.g., 90% or more); a copper foil defined for the degree of development of the cubic texture after recrystallization annealing (e.g., the intensity of a (200)Cu plane determined by X-ray diffraction at the rolled surface is greater by more than 20 times than that determined by powder X-ray diffractometry); a copper foil defined for the ratio of penetration crystal grains in the direction of thickness of the copper foil (e.g., 40% or more as a cross sectional area ratio); a copper foil controlled for the softening temperature by the addition of small amount of additive elements (e.g., controlled to a half-softening temperature of 120 to 150° C.); a copper foil defined for the length of a twin boundary (e.g., the total length of the twin boundary with a length exceeding 5 μm per 1 mm2 area is 20 mm or less); a copper foil controlled for the recrystallization texture by the addition of additive elements (e.g., the Sn is added by 0.01 to 0.2 mass % to control the average crystal grain size of 5 μm or less and the maximum crystal grain size of 15 μm or less), etc. (see JP-B-3009383, JP-A-2006-117977, JP-A-2000-212661, JP-A-2000-256765, JP-A-2001-323354, JP-A-2001-262296, and JP-A-2005-68484).
However, along with development in downsizing, increase in the integration degree (higher density mounting) and higher performance of electronic equipment in recent years, further higher requirement for flexible fatigue property has been increased more and more than usual for the FPC. Since the flexible fatigue property of the FPC is determined substantially depending on that of the copper foil, it is essential to further improve the flexible fatigue property of the copper foil for satisfying the demand.